Kinetic analysis of thermally induced polymorphic transition in HMX using terahertz spectroscopy

Abstract

Understanding the kinetics of polymorphic transitions is critical for predicting and controlling phase behavior in functional materials. Although terahertz spectroscopy has been established as a reliable technique for identifying polymorphic forms, its application in analyzing transition kinetics remains underexplored. In this study, terahertz time-domain spectroscopy (THz-TDS) was employed to monitor the thermally induced β-to-δ polymorphic transition in HMX. Principal component analysis enabled segmentation of the spectral evolution, allowing precise identification of intermediate transformation stages. Density functional theory calculations, combined with analysis of thermally activated anharmonic vibrational modes, revealed that nitro-group motion facilitates the transition by lowering activation barriers. A logistic relationship between absorption intensity and transformation time was observed, consistent with a cooperative nucleation-and-growth mechanism governed by thermal input. These results demonstrate the capacity of THz spectroscopy to dynamically monitor polymorphic transition kinetics at the molecular level.